The present invention relates to an improved mesh cell design for a trawl system (that by definition is iterated or cloned in varying geometric patterns) providing improved shaping and performance, especially when incorporated in mid-water or bottom trawls of such systems.
It is well understood that the basic cell of a selected portion of every trawl system is the unit cell (called mesh cell hereinafter). The selected portions of the trawl system is then built by repeating the shape of the basic mesh cell.
It is axiomatic that the ability to predict the overall shape and performance of the finished product depends entirely on the shape and structural integrity of that single basic mesh cell. Heretofore, proper trawl making was a two-step process that involved initial construction of undersized mesh cells, and setting the knots and mesh sizes by the substeps of depth stretching and heat setting involving turning the finished mesh in direction opposite to its natural bent and applying first pressure, and then heat to set the knots.
Materials used in mesh cell construction can be plastics such as nylon and polyethylene but other types of natural occurring fibers also can be (and have been) used. Single, double (or more) strands make up a thread or twine composed of, say, nylon, polyethylene and/or cotton. Additionally, in making the mesh portion of conventional trawls particularly mid-water trawls especially the forward section mesh portion thereof, braided cords and twisted ropes of natural and synthetic materials, bonded and unbonded, and cables have been used. However, the pitch of any braided or twisted thread, such as a twine, cord and/or rope (distance between corresponding points along one of the strands constituting one turn thereof which is analogous to the pitch between corresponding screw threads) either has usually been small, or has produced shallow or narrow depressions. Conventional trawl making practices balance the towing force generatable by a vessel against the largest possible trawl for a particular fishing condition, i.e. a trawl having the minimum possible drag. Thus, conventional trawl makers are taught to use the smallest possible diameter twine to reduce drag. Accordingly, meshes in conventional trawls, and especially the mesh of the forward sections of mid-water trawls, have been made of twines, including conventional three strand twisted twines of any pitch including loose pitch, that have relatively shallow or narrow and uniform spiral depressions, or smaller diameter braided twines having an equivalent breaking strength. Moreover, modern manufacturing processes using threads, such as twines, cords, cables or ropes to form mesh cells, have always been designed to produce mesh cells in which twist direction of the individual bars comprising each mesh cell, if any, is always the same. None have proposed the systematic and regular use of differently oriented twist for individual mesh bars of the mesh cell in the manner of the present invention.
Even though various Japanese Patent Applications superficially describe mesh cells for nets in which mesh bars have differing lay directions, (see for example, Jap. Pat. Apps. 57-13660, 60-39782 and 61-386), the mesh bars employ conventional, essentially smooth twine or rope. The patent applications disclose differing lay directions of conventional, essentially smooth twine or rope for balancing residual torque within the net structure during its deployment and use, not for generating lift that enhances of trawl system performance. The first-mention Application, for example, states that its purpose is to provide xe2x80x9cnet legs with different twist directions according to a fixed regular pattern so that torsion and torque of said net legs are mutually canceled.xe2x80x9d The use of conventional, essentially smooth twine or rope will not yield substantial lift any different from conventional nets.
As set forth in published Patent Cooperation Treaty (xe2x80x9cPCTxe2x80x9d) International Patent Application, International Publication Number WO 97/13407, International Publication Date Apr. 17, 1997, (xe2x80x9cthe PCT patent applicationxe2x80x9d) it has been recently discovered that threads, such as twines, cords, braided cords, cables, ropes or straps, may be advantageously twisted, during assembly of trawl net meshes into a loose, corkscrew-shaped pitch establishing helical grooves that are deeper and/or broader than the depressions in conventional tightly or loosely twisted multi-strand ropes or cables making up conventional mesh bars. During field operations in a water entrained environment, properly orienting mesh bars having the loose, corkscrew-shaped pitch produces lift that increases a performance characteristic of a trawl system such as increased trawl volume (particularly in shallow water) in comparison with a trawl made from conventional mesh, improved trawl shape, and reduced vibration, noise, and drag. Trawl performance improves even though, contrary to conventional trawl design, mesh bars having the loose, corkscrew-shaped pitch have a diameter (or shadow area) larger than corresponding mesh bars of a conventional trawl.
An object of the present invention is to provide further improved trawl systems.
Yet another object of the present invention is to provide trawl systems having improved performance characteristics.
Briefly, the present invention improves upon the basic discovery disclosed in the PCT patent application that individual bars of a mesh cell can be formed to act as mini-hydrofoils in field operations. During field operations in a water entrained environment, the trawl disclosed in the PCT patent application becomes disposed symmetrically about a central axis. The disclosed trawl includes a plurality of mesh cells, each mesh cell having at least three mesh bars. Each mesh bar in the trawl intersects with at least one other mesh bar. During field operations with the trawl in a water entrained environment, at least a portion of at least one of the mesh bars of at least one of the mesh cells in the trawl exhibits a substantial hydrofoil-like effect that aids in increasing a performance characteristic of a trawl system. That portion of mesh bars in accordance with the present invention which generate a substantial hydrodynamic lift is preferably formed from a material that has a substantially incompressible cross-sectional shape, is offset from the central axis of the trawl, and is formed with a hydrofoil shape that:
1. has a lay with a loose, corkscrew-shaped pitch establishing a corkscrewing groove that provides cambered sections; and
2. is oriented to establish leading and trailing edges for that portion of mesh bars which generate the hydrodynamic lift.
The lay of that portion of mesh bars which generate the hydrodynamic lift has an orientation relative to a receding direction, and the leading edge for that portion of mesh bars which generate hydrodynamic lift, when normalized to the receding direction relative to the central axis, resides at a side of the mesh bar. Pairs of lay and the leading edge are selected for mesh bars from a group consisting of:
1. a left-hand lay, and the leading edge being a right side of the mesh bar as viewed in the receding direction; and
2. a right-hand lay, and the leading edge being a left side of the mesh bar as viewed in the receding direction.
If the lay and leading edge are selected from the preceding group, then movement of the mesh bar in accordance with the present invention through the water entrained environment relative to a water flow vector that is neither parallel nor perpendicular to the mesh bar creates a pressure differential across that portion of mesh bars which generate hydrodynamic lift. The pressure differential thus created across such mesh bars establishes a lift vector relative to the central axis of the trawl, most commonly directed away from the central axis of the trawl. Consequently, the lift vector created by movement of the mesh bars which have a portion that generated hydrodynamic lift increases the performance characteristic of the trawl which is selected from a group consisting of substantially increased trawl volume (particularly in shallow water) in comparison with a trawl made from conventional mesh, improved trawl shape, and reduced vibration, noise, and drag.
Various other aspects of the present invention further improve the performance of trawl systems. Thus, properly controlling the shape, arrangement, and distribution of strands assembled to form a mesh bar further improves a trawl in accordance with the present invention. The proper pitch for the loose, corkscrew-shape is advantageously controlled so the pitch of each mesh bar is in a range of 3d to 70d, with a range of 5d to 55d being preferred, where d is:
1. for a pair of twisted strands forming a mesh bar, the diameter of the smaller strand of the pair;
2. for mesh bars that include more than a pair of twisted strands or strands of differing diameters, the diameter of the next-to-largest diameter twisted strand; or
3. for straps forming a mesh bar, the width of the strap.
Within the preferred pitch range, a pitch of 5d to 15d generally produces maximum lift for mesh bars formed from product strands, while a pitch of 25d to 55d generally produces minimum drag for mesh bars formed from product strands.
For mesh bars formed by straps, a pitch of 8d to 30d generally produces maximum lift, while a pitch of 9d to 21d generally produces minimum drag. A ratio for the width of the strap to a thickness of the strap is preferably in a range of 1.5:1 to 20:1. Straps for which the ratio is in a range from 2.5:1 to 2.75:1 provide both low drag and good lift. Straps for which the ratio is in a range from 2.75:1 to 10.0:1 provide high lift. Straps for which the ratio is in a range from 1.8:1 to 2.5:1 provide low drag with good lift. Straps for which the ratio is in a range from 1.5:1 to 1.8:1 exhibit lower drag. Straps in the range from 1.8:1 to 2.5:1 may be used advantageously in the mid-section and/or back-end of the trawl. Conversely, straps in the range from 2.75:1 to 10.0:1 may be used advantageously in the front-end, particularly near a leading edge of the trawl.
These and other features, objects and advantages will be understood or apparent to those of ordinary skill in the art from the following detailed description of the preferred embodiment as illustrated in the various drawing figures.
MESH is one of the openings between threads, ropes or cords of a net.
MESH CELL means the sides of a mesh and includes at least three sides and associated knots or equivalent couplers oriented in space. A quadratic mesh cell has four sides with four knots or couplers, and is usually arranged to form a parallelogram (including rectangular and square), with diamond-shaped mesh (trawl mesh) being preferred. A triangular mesh cell has three sides and three knots or couplers. A hexagonal mesh cell has six sides and six knots or couplers.
MESH BARS means the sides of a mesh cell.
CELL means a trawl construction unit, fishing net or the like and includes both a mesh cell relating to enclosable sides of the mesh of the trawl or net itself, as well as to upper bridle and frontropes used in towing the trawl or net through a water column to gather marine life.
CELL BAR means both the sides of a mesh cell and the elements that make up the upper bridle, frontropes and tow lines.
RIGHT- AND/OR LEFT-HANDEDNESS IN A RECEDING DIRECTION along a cell bar involves establishing a central axis for the trawl, net or the like to which the mesh cell associated with the cell bar belongs. Then a normalized imaginary giant stick figure, that is depicted in FIGS. of the PCT patent application, is positioned so his feet intersect the central axis, are rotatable about the central axis, his body penetrates through the cell bar, and his back is positioned perpendicular to and first intersects the water flow vector for the moving trawl, net or the like. The right- and/or left-handedness of the cell bar is then determined using the location of either his right or his left arm irrespective of the fact that the position of the cell bar is offset from the central axis.
THREADS are composed of synthetic or natural fibers. Firstly, for the present invention a thread can comprise two strands twisted along the longitudinal axis of symmetry in a loose fashion with a pitch in a range of 3d-70d, where d is:
1. for a pair of twisted strands forming a mesh bar, the diameter of the smaller strand of the pair; or
2. for mesh bars that include more than a pair of twisted strands or strands of differing diameters, the diameter of the next-to-largest diameter twisted strand.
Or secondly, for the present invention a thread can comprise a extruded, woven, braided, or plaited strap that is twisted along its longitudinal axis of symmetry in a loose fashion with a pitch in a range of 3d-70d, where d is the width of the strap.
STRAP is a flexible element of synthetic or natural fibers that forms a mesh bar, the strap having a cross-section that is generally rectangular or can be quasi-rectangular with rounded short sides and elongated long sides with or without camber. In operation, the strap acts as a hydrofoil, preferably twisted along its longitudinal axis, wherein the short sides form inter-changing leading and trailing edges.
PRODUCT STRAND includes the synthetic or natural fibers or filaments used to form the construction unit of the invention which is preferably, but not necessarily, the product of a conventional manufacturing process, usually made of nylon, polyethylene, cotton or the like twisted in common lay direction. Such strand can be twisted, plaited, braided or laid parallel to form a sub-unit for further twisting or other use within a mesh bar or a cell bar in accordance with the invention.
NET is a meshed arrangement of threads that have been woven or knotted or otherwise coupled together usually at regular intervals or at intervals that vary usually uniformly along the length of the trawl.
TRAWL is a large net generally in the shape of a truncated cone trailed through a water column or dragged along a sea bottom to gather marine life including fish.
CODEND is a portion of a trawl positioned at the trailing end thereof and comprises a closed sac-like terminus in which the gathered marine life including fish are trapped.
FRAME is a portion of the larger sized meshes of a net or trawl upon which is overlaid (and attached by a binding) a netting of conventional twist.
PANEL is one of the sections of a trawl and is made to fit generally within and about frames shaped by riblines offset from the longitudinal axis of symmetry of the trawl.
PITCH is the amount of advance in one turn of one product strand twisted about another product strand (or strands) when viewed axially, or common advance of the twist of a strap along its axis of symmetry. For product strands, pitch values are determined with respect to the diameter of the next-to-largest product strand. For straps, pitch values are determined with respect to the width of the strap.
LAY is the direction in which the strands or the strap wind when viewed axially and in a receding direction.
INTERNAL LAY OR TWIST is the direction in which synthetic or natural fibers comprising each product strand are wound when such strand is viewed axially and in a receding direction.
INTERNAL BRAID describes the method of formation of a particular product strand.
FRONTROPE(S) is a term that includes all lines located at perimeter edge of the mouth of the trawl, net or the like, such as headrope, footrope (or bottomrope) and breast lines. The frontropes have a number of connections relative to each other and to the bridle lines.
BRIDLES relates to lines that intersect the frontropes and attach to the tow lines. For a particular port or starboard tow line, a pair of bridles extend from a common connection point therewith, back to the frontropes.
TRAWL SYSTEM is a term that includes the trawl, net or the like in association with the tow lines therefor as well as the bridles lines.